1. Field of the Invention
The present invention relates to a system for remotely determining the velocity of sound in water, and more particularly to a system using range resolved Brillouin scattering with a pulsed laser.
2. Description of the Prior Art
The measurement of sound velocity in water, especially the ocean, is now done by using various expendable and non-expendable in-situ sensors. It is desired to build a remote sensing instrument to supplement these previous methods and give wide area coverage, and which can map sound velocity in both the horizontal and vertical directions.
When a beam of monochromatic light passes through a transparent fluid, inhomogeneities in density produce scattering. Inhomogeneities in density which are in motion will produce a shift in the frequency of the scattered light due to the Doppler effect. If the inhomogeneities are periodic, and they satisfy the Bragg equation EQU .lambda..sub.op =2n.lambda..sub.ac cos .theta. (1)
where .lambda..sub.op is the wavelength of the light, n is the index of refraction of the medium, .lambda..sub.ac is the wavelength of the inhomogeneity and .theta. is the angle between the incident light wave and the axis of propagation of the periodic inhomogeneity, there will be a maximum in the scattering due to contructive interference between the photons scattered from each layer of density inhomogeneity.
Sound waves are characterized by such periodic inhomogeneities in density. Brillouin predicted the magnitude of the shift due to sound waves to be ##EQU1## where .omega..sub.B is the Brillouin shift in frequency, C.sub.o is the speed of sound in the medium and .theta. is the angle between the incident and the scattered light wave vectors. Almost since its inception the CW gas laser has been used in the measurement of Brillouin scattering. The advantage of the CW laser is its monochromaticity and narrow line width. The disadvantage in using the CW laser in a remote sensing instrument is that range resolution must be achieved by bistatic or focusing techiques which are slow and suffer from decreasing range accuracy with increasing range. If Brillouin scattering could be used, the desired remote sensing instrument could be built.